The Sun It's true the hazards of Sun observing have probably been overstressed in amateur literature, needlessly scaring people out of viewing the Sun, but the danger is real. A moment's carelessness can result in a lifetime of damaged eyesight. If the Sun is treated with respect, though, it can be a rewarding subject for observation and study.
What's visible on the Sun? As mentioned in the solar filter discussion, an inexpensive white-light filter will mostly show sunspots and the granular structure of the Sun's surface, the "faculae." These things can be endlessly fascinating, but most Sun-worshiping amateurs wind up wanting more. A hydrogen alpha filter is an expensive buy for most amateurs, but the dim red light of hydrogen that passes through it can reveal the Sun's more dramatic secrets, including prominences, the glorious fountains of fire that ring the Sun's disk. Hydrogen alpha filters are complex and hard to make and will probably never be truly inexpensive, but Lumicon's Solar Prominence filter system is at least palatable, with the 8-inch SCT model going for $780.
The biggest problem for the prospective solar observer? Seeing. Whether trying to view or image the Sun, daytime atmospheric turbulence is always a problem. Not only is the daytime atmosphere usually in turmoil, the scope is "shooting" over Sun-warmed ground, and its tube is sitting in the full Sun and filled with currents of hot air. The most effective way to deal with solar seeing is to observe in the early morning, just as the Sun attains 30 degrees of altitude over the horizon. At that time the atmosphere will be at its steadiest, Earth will not yet be overly warmed, and the rays hitting the telescope OTA will be relatively gentle.
Mercury Mercury, named after the fleet-footed messenger of the gods, is the first stone from the Sun, orbiting at a distance of about 58,000,000 km, making it swiftest member of Sun's family. With a diameter of less than 5,000 km it is also the smallest major planet. Being close to the Sun, Mercury is never far from its master in the sky. As it swings around in orbit, it appears as a "morning star" before dawn on one side of its orbit and as an "evening star" after sunset on the other side. At its greatest "elongation," its greatest distance from Sol, Mercury is no more than about 30 degrees from our blinding star. Given its small size and considerable distance from us, Mercury is understandably small in telescopes, no more than 5 to 7 seconds of arc in diameter.
What can an SCT owner see of Mercury? Before seeing anything, Mercury will have to be found. That's not usually a problem, even without go-to. Despite a reputation for being elusive, Mercury stands out like a sore thumb at observing sites with uncluttered horizons. People who've never seen the planet are often amazed at how prominent the little guy is. Mercury can get as luminous as magnitude -1.9 and appears as an unmistakable yellowish "star" lurking near the horizon. Locating Mercury is one thing; seeing much of this distant, rocky world in a telescope is another.
How much Mercurian detail is visible in an 8-inch or larger SCT? Little or none. Being so close to the Sun, it can only be viewed down in the thick, dirty, turbulent air near the horizon. An "inferior" planet (closer to the Sun than Earth), Mercury goes through phases, just like the Moon, growing from a slim crescent to almost full (the Sun always hides a "full Mercury"). That's about all most amateurs will ever see of this planet: a tiny Moon-like thing that moves swiftly into and out of the solar glare. Can anything help with Mercury? Mainly, just observing the planet when it's as high in the sky as possible. That means catching it just after sunset in the evening and just before sunrise in the morning. At those times the background sky is admittedly bright, but eyepiece filters can help with that. Red and orange are particularly good for darkening the sky and increasing contrast between Mercury and the background. Some observers have had good results by observing Mercurywhen the Sun is over the horizon. It's easy enough to view the planet in the daylight by continuing to watch it as the Sun rises, or, if it's in its evening star role, by enlisting the aid of a go-to system to locate it. The important thing? Extreme care must be taken not to get the Sun in the field by accident!
Images delivered by the Pioneer 10 spacecraft revealed Mercury's surface as a crater covered landscape similar to the Moon. Can hints, at least, of these craters be seen from Earth? It does not appear so—not visually, anyway. Over the years, visual observers with a variety of telescope types have recorded dusky markings on the planet, but these do not seem to correspond to real features in spacecraft images. Recently, however, amateurs using webcams (Chapter 11) have produced pictures of the planet with features that seem to tally with Pioneer images and those returned by the recent (2008) Messenger spacecraft.
Venus The next planet out from the Sun is Venus. Aphrodite's beautiful appearance in morning and evening skies, where she outshines everything except the Moon and Sun, leads novices to expect great things from her. I remember how excited I was to get my first look at the planet through a telescope. What wonders would be on display? Venus had, up until the end of the 1960s, almost as romantic a reputation as Mars, being imagined as a watery ocean-covered world or a steamy swamp-dominated planet, perhaps inhabited by dinosaurs. What would I see in my 3-inch telescope?
Not much. Through a small instrument, and indeed through any telescope, Venus turns out to be another disappointment. It is really just a larger and brighter version of Mercury, a featureless disk that, due to its status as an inferior planet, shows phases like the Moon. The bland nature of Venus despite its close proximity to Earth is due to a deep blanket of clouds. The spacecraft that began visiting the planet in the 1960s found Venus suffers from a terminal case of runaway greenhouse effect. The carbon dioxide-laden atmosphere traps heat, resulting in a surface temperature of about 900 degrees Fahrenheit. Goodbye Venusian dinosaurs and mermaids.
Is Venus a complete waste of time for an SCT user? Not necessarily. It is much less interesting than Jupiter, Saturn, or Mars, but there are things to see. It's fun to watch Venus's phases and see it grow from a small gibbous disk to a large, thin crescent and vice-versa. There's also the ashen light. If you've ever admired a lovely crescent Moon, you've no doubt noticed you can see not only the Sun-illuminated portion but also the night side glowing feebly. The reason the dark part of the Moon's disk is visible is simple: a bright Earth is in the lunar night skies illuminating the landscape with reflected sunlight, just as a full Moon lights-up the landscape of our own world. Over the years, quite a few Venus observers, including your author, have noted a similar effect on that planet. In addition to the illuminated part of the disk, the dark portion of Venus can sometimes also be seen faintly. But how is that possible? Venus has no Moon to light its evenings!
Nevertheless, the ashen light must be real. I myself saw it convincingly for the first time in 1999 with an 8-inch Schmidt Cassegrain. I'd imagined I'd seen the effect occasionally a time or two before that, but was never was completely sure. On this particular night there was no doubt. The night side of the half-illuminated planet was remarkably visible. The faint yet obvious glow remained visible even when an 80A blue filter was added to the eyepiece.
What is the ashen light? It is a real effect, but it may not necessarily be a real phenomenon of the planet. The human eye/mind is a wondrous combination, but is all too prone to showing us what we expect to see. Venus looks like a little Moon, so the brain delivers a little Moon image, complete with Earthshine. Combine this "fill in the blanks" characteristic of the eye/brain with effects caused by the high contrast between the brilliantly illuminated planet (as bright as magnitude -4.6) and the sky, and we don't have to look to Venusian aurorae for the cause of the ashen light. The ashen light "case" is far from closed, however.
An even higher challenge for the visual observer is Venus's markings. By markings, we mean shadings in the planet's impenetrable atmosphere caused by clouds, not anything on the surface. Don't imagine these atmospheric features will stand out like the cloud bands of Jupiter. They are incredibly faint and subtle. Occasionally faint patches can be noticed along the terminator, but these can usually be ascribed to contrast effects. A #47 violet eyepiece filtercan help some, but seeing the Venusian clouds is still close to hopeless. It is possible to record details in the atmosphere of the planet easily enough. A webcam, especially one equipped with a filter that passes UV and blocks most other wavelengths, will definitely show shadings in Venus's steaming atmosphere.
How often should you observe the lovely lady? Maybe a few times per Venus apparition. For most SCT users, Venus is a featureless blank of a world, an object of occasional interest rather than a lifelong obsession, like the "big three," Mars, Jupiter, and Saturn. This is not to say Venus does not have her fans. The amateur organization for planetary observers, ALPO, the Association of Lunar and Planetary Observers, has an active Venus section.
Mars Mars has fascinated astronomers for centuries. After Jupiter, it is probably the most interesting planet for SCT owners. Unlike Venus, Mars offers detail aplenty: subtle but easily visible surface markings that sometimes change, polar ice caps that grow and shrink with the planet's seasons, atmospheric clouds that come and go, planet-girdling dust storms, and more. Plus, there's the simple fascination of Mars as a place that keeps us coming back. Though we now know Mars is not the "abode of life" that wealthy American amateur astronomer Percival Lowell imagined, it's possible life (most likely microbial life) existed there in the distant past. It's even conceivable there's still some primitive life lurking on this small (6,800-km) world. There is little doubt Mars was once much warmer and much wetter.
Because of these things, it's no surprise Mars is one of the first targets to attract the attention of a new CAT owner. Unfortunately, the novice often feels short-changed. Mars can be fascinating for the visual observer equipped with a small telescope, but it is usually just plain difficult. Why? Because it is small and far away. Mars, barely more than half the size of Earth, orbits the Sun at a distance of about 225 million kilometers. In some parts of its orbit it can be almost 400 million kilometers away from Earth. At that distance it's so tiny in the telescope it's barely worth a glance. Even large instruments show little or nothing of its surface features. If that were all there were to the Mars story, it would probably elicit less interest from amateur astronomers than even bland Venus. But that's not the whole story. Every two years, there's a magical Mars Time.
Every other year Mars comes to "opposition," the point in its orbit when Earth is directly between it and the Sun. At that time it is closest to Earth and directly opposite the Sun in our sky, making it well placed for telescopic observation all night long. The distances of Mars' closest approaches depend on exactly where it is in its fairly eccentric elliptical orbit at opposition time. Every 15 to 17 years, though, it comes really close. The 2003's Mars opposition, when the planet was a "mere" 55,768,000 kilometers from Earth, was the closest in 60,000 years. At that opposition Mars was over 25 arc seconds in diameter and shone at a magnitude of -2.7. That is remarkably big and bright for this planet, and, as might be expected, the amateur images and visual observations done at the time revealed amazing detail (Plate 51). Sorry if you missed it, since it won't be that good again until 2287, but even a not-so-close opposition (in 2010 Mars will be 99,000,000 km away) is a fine time to be a Mars watcher equipped with an 8-inch or larger CAT.
Plate 51. (Mars) Mars by a C8 and a webcam during the amazingly close 2003 opposition. Credit: Author.
As an opposition approaches, the tiny red speck of a planet grows and grows, and the normally featureless disk begins showing more and more detail, the legendary dark patches and ice caps popping into view. At opposition the magnitude of Mars also increases, making the normally sedate planet positively glaring. During these times it seems as if every telescope on Earth is staring at the Red Planet.
The Martian polar ice caps and dark ("albedo") features are easy even in a 4- or 5-inch CAT when the planet is at opposition. Depending upon where the planet is in its orbit, either the north polar ice cap or the southern one may be pointed in our direction, and will be easy to make out as an intensely bright white spot. As the seasons change, the visible polar cap grows and disappears. Larger CATs may show that the cap changes shape as it grows and shrinks, and may even reveal a dark "melt" line adjacent to it as summer comes in. The dark surface features of the planet are what really draw observers, though. There are no canals, but their lack is more than made up for by the maria of Mars. These subtle dark patches were once thought to represent vegetation but are now known to be nothing more than areas of the planet that have been scoured clean of dust by Martian winds. The dark areas bear watching by amateurs because they can change subtly, and this is of interest to planetary scientists. The easiest of these dark features to identify, a true Martian landmark, is the "Indian Subcontinent of Mars," Syrtis Major, the large, dark area centered in the middle image of Plate 51.
Even at opposition time, there's no denying Mars is small (less than half the average size of Jupiter), and that picking out surface features can be tough. Don't be afraid to use high magnification. It's always easier to pick out details in a larger image than a smaller one, even if more magnification makes the planet less sharp. One other thing that will help is a filter. A red or orange filter (Wratten #21 orange or #25 red) works, but even more contrast can be achieved with the peach-colored "Mars filters" astro-vendors sell around opposition time. The biggest help, though? Experience. After a few weeks of observing the planet at high power, details that were initially hard to make out become easy.
Mars is a fairly dynamic world, and changes in its atmosphere happen on a regular basis. It's not unusual for its clouds and weather systems to be prominent enough to be seen in an 8-inch SCT. In addition to yellow clouds that represent dust storms, there are frequent blue clouds caused by planetary weather systems. A large dust storm can be obvious in a 90mm ETX, while the blue clouds of Martian cold fronts may need a big CAT and an experienced observer. As with the maria, filters can help with clouds. Try green or blue for fronts and hazes and yellow for dust-storm clouds.
How about Mars' two moons, Phobos(fear) and Deimos(panic)? These asteroid-sized chunks of rock require at least an 8-inch SCT. At opposition they are not that tough—or wouldn't be if it weren't for the overwhelming glare of the planet. The magnitudes of the pair are not overly dim; Phobos gets as bright as magnitude 12, while Deimos is about 13. Despite Phobos's relative brightness, it's the more difficult of the two, since it's closer to the planet. The trick to seeing these two worldlets is to either get Mars out of the field or arrange some kind of "occulting bar" in the eyepiece— maybe a strip of aluminum foil taped across the telescope end of the eyepiece. Hide Mars behind this bar, and one or both of the minute moons may pop into view. Before trying to observe the moons, find out when and where they will be at their greatest distances (elongations) from the planet, which is when they'll be easiest to find. Times/dates of Phobos' and Deimos' elongations can be found on the Internet or in the astronomy magazines.
Mars, riding high in the night sky at opposition, is a magnet for amateur astronomers. For those of us who dream of traveling there or even of colonizing this strange world, our little CATs provide us with a unique opportunity to visit this fabled and secretive world vicariously. Sadly, few, if any, people now living will have a chance to actually walk the sands of Mars; that appears to be an honor reserved for our grandchildren or great grandchildren. But our faithful CATs allow us to travel there in spirit and taste a few of the wonders of humankind's most likely second home.
Jupiter Jupiter (Plate 52) is the king of the planets and not just because of his enormous girth—this monster of a world is 142,984 km in diameter. Jupiter is also king in the affections of amateur planetary observers. Why? Because this great ball of gas is just so consistently interesting. There's always something to see any time Jupiter is in the sky. Even a little MCT reveals multi-colored cloud bands. The Great Red Spot, the planet's enormous storm system, cruises sedately around the planet, drawing the eye of the telescopic observer. Accompanying the planet are the four huge "Galilean" moons discovered by Galileo on the night when he first turned his telescope to the sky. These satellites shuttle back and forth from evening to evening, crossing in front of Jupiter's disk (transits), casting dark, pinpoint shadows on the planet's cloud deck (shadow transits), and disappearing behind the globe (eclipses). And Jupiter is not just interesting because of the wealth of detail on its huge (45 arc second diameter) disk but because these details are always changing.
Mars is fascinating, sure, but fans of the Red One have to wait for every-other-year oppositions before being able to see much. In contrast, Jupiter, although more distant than Mars, orbiting 778,330,000km from the Sun, is so big that it is a worthy target for CATs anytime it is visible, which is for months at a time every single year. Jupiter does vary a bit in size, but never gets small, and details are always easy to discern even with small apertures and low magnifications. For example, although
Plate 52. (Jupiter) Jupiter presents a mass of detail both to the camera and to the naked eye." Credit: Author.
the Great Red Spot is currently (2008) a fairly pale pinkish-red, itvcan be seen with a 90mm inch ETX. Imagine the kind of detail visible in an 8- to 10- or 12-inch SCT. On a night of good atmospheric seeing, the features on Jupiter visible in an 8-inch CAT are almost mind-boggling. Not only are there four or five dark cloud belts but also a wealth of detail in these belts, ranging from ragged edges to streamers (festoons) impinging into the bright "zones" that separate the dark bands. When the Great Red Spot is undergoing one of its darker periods, you can detect not just its oval shape but tantalizing hints of detail within it.
The four Galilean moons, one of which is visible in Plate 53, also put on a wondrous display in a C8. When the atmosphere is at maximum steadiness, they are not just star-like points; they show tiny perfect disks at high magnification (the largest, Ganymede, at 5,262 km is planet-sized). It's easy to spot the hard little shadows these moons cast on Jupiter's cloudtops as they transit in front of the disk. Under stable seeing conditions it's even possible to track a satellite itself as it moves across the planet. The moon will appear as a tiny, bright disk set against the darker background of mighty Jove.
Some beginners wonder why their SCTs show only four moons despite the fact that Jupiter is known to have a huge retinue of at least 62 satellites. The reason is that the other moons are all tiny and dim. They are flying mountains rather than small worlds. The brightest of them, Amalthea, the last of Jupiter's companions to be discovered visually (1892), is a dim magnitude 14.1. When this is coupled with the moon's nearness to the bright disk of the planet, Amalthea becomes a terribly challenging object, even for the largest amateur SCTs. Think "Phobos and Deimos" but much worse.
Jupiter is immediately impressive to the new SCT owner, but beyond cloud bands and the moons, not much will be visible initially. Making out detail on Jupiter is easy compared to the difficulties the other planets present, but it still requires experience and knowing a few tricks of the trade. Foremost—as with all the planets—is the requirement for precise collimation of the telescope. That makes all the difference in the world with SCTs. Magnification? Jupiter, being larger, doesn't need as much as
Plate 53. (Jupiter and Satellite) Distant Jupiter's largest moons show their disks to webcam-equipped SCTs and in high power eyepieces on steady nights. Credit: Author.
Mars. The Great Red Spot and small atmospheric features such as spots and festoons are often detectable at powers of 100x by experienced observers, and 200x is often more than enough magnification to reveal smaller features. As always in the planetary game, however, don't be afraid to kick the power up a couple of notches.
Despite the planet's many wonders, new Jupiter observers are often let down by the subdued colors of the planet. It doesn't look a thing like the Voyager spacecraft pictures. It's all washed out. Where are the dark reds and bright blues and fluorescent yellows? The Voyager pictures, while amazing, are not a realistic representation of the planet's appearance. Contrast and color saturation in spacecraft images were boosted in order to make small details more visible. Jupiter, the real planet, seen live in the eyepiece, is not a riot of color; it's a pastel world. Colors are visible to the experienced observer, but they tend to be muted tans, creamy-yellows, and subtle blues, not strong primary colors.
Does Jupiter seem a bit too pastel to make it easy to see belts, spots, and festoons? There's a way to enhance the Jupiter experience: a Wratten #80A blue filter. This probably does more to enhance the planet than any other filter on any other planetary subject. An 80A sometimes makes the difference in seeing and not seeing barely visible belt features. As we said earlier, the 80A is indispensable for Jupiter watching. A few other filters can help a little, too. A yellow one (Wratten #12 or #15) will darken the festoons. A magenta filter (#30) can help with bright spots and ovals.
This author been observing Jupiter for over 40 years, and the planet never fails to amaze. Just when you think you've seen it all, something dramatic happens. The Great Red Spot fades, belts disappear and reappear, long-lived white spots bloom and cruise and collide and merge, or a comet slams into the planet with incredible violence. This enormous world, almost frightening in its majesty, reminds one that our Solar System is not a static thing but an entity that changes and lives. The truly wonderful thing is that even the tiniest CAT provides a ring-side seat for Jupiter's ever changing and never ending show.
Saturn Saturn, Lord of the Rings, is without doubt the most beautiful object in the heavens. A first glimpse of this almost artificial-looking world is unforgettable. It's just too perfect to be believed. Guests at public star parties peer down the corrector end of a scope after viewing Saturn, looking for the photo of the planet they're sure was pasted on the end of the telescope!"
Beyond the striking beauty of Saturn's golden-orbed, ringed visage, there's a fair, if not overwhelming, amount of detail for CAT owners. Examine the rings carefully, even with a 90mm ETX, and a thin black line dividing Saturn's "A" and "B" rings (the outer and inner rings, respectively) will be obvious. This is the Cassini division, named for the seventeenth-century astronomer who first noted this curious feature. It is caused by gravitational effects that sweep this area clean of ring particles. The rings, of course, are not solid but are composed of pebble- to mountain-sized chunks of ice. The Voyager spacecraft revealed numerous other gaps in the rings, all of them much narrower than Cassini's. The only one of these other ring divisions visible from Earth lies almost at the edge of the "A" ring and is called the "Encke" or "Keeler" gap.
Over the years, numerous amateur observers who reported a division in that spot at the edge of the rings were met with skepticism on the part of the pros. Finally, the Voyagers put the question to rest in 1980 and 1981, imaging the Encke Gap clearly. Occasionally one can catch a glimpse of it in an 8-inch SCT at obscenely high powers on the best nights, but what most small scope owners may actually be seeing is the Encke "Minima," a slight darkening of the A ring near its outer edge rather than the gap itself. Even this "Minima" not overly easy to see, even with a webcam. Plate 54 shows it but just barely.
Inward from the "B" ring is Saturn's final major ring, the "C," or Crepe, Ring. The Crepe is somewhat difficult to see in small telescopes. It is semi-transparent and appears as nothing more than a faint haze inside the "B" ring. Often the easiest way to detect this subtle band is to look for a darkening where the ring passes in front of the planet.
Like Jupiter, Saturn is a gas giant world, a great ball of (mainly) hydrogen with (perhaps) a small rocky core at its center. The appearance of its globe is very different from that of Jupiter, however. Jupiter is a pastel low-contrast world, but the cloud features on Saturn are even more understated. Because of what is apparently a hydrocarbon haze high in the atmosphere, Saturn's belts, spots, and zones are lower in contrast than those of Jupiter. Most obvious is a bright equatorial zone. This is flanked on either side by tan north and south equatorial belts that are fairly easy to detect against the burnished gold of Saturn's globe. Other belts can be seen higher in latitude in either hemisphere, but the narrow, subdued zones that separate them make it difficult to distinguish one from another. On particularly steady nights, 8-inch and larger CATs may be able to detect a faint darkening around the pole of the planet that is currently pointing towards Earth.
Not only is Saturn's atmosphere lower in contrast than Jupiter's, it's less active, probably because the planet, a whopping 1.35 billion kilometers from the Sun, is colder. Bright spots can occasionally be seen, but they are far more difficult to observe than comparable features on Jupiter. These spots seem to be associated with Saturn's closest approaches to the Sun (perihelion) during its 29-year orbit and are
Plate 54. (Saturn)
Saturn is beautiful but (almost) unchanging. Credit: Author.
usually found near the equatorial zone. Frankly, the best way to "see" Saturn's spots is with a webcam; even then they are not exactly prominent.
What special problems does Saturn present to the SCT owner? The main trouble with Saturn is its huge distance from the Sun. New observers, once they get over the initial impact of Saturn's beauty, are distressed by how tiny it is. At opposition, Saturn's disk is about half the size of Jupiter's, and to see many details in the rings or on the globe high power is required. The 200x is a good "Jupiter power," but this is merely the starting place for Saturn. Luckily, the planet seems to "take" magnification better than Jupe. You can use as much as 600x on Saturn with a C8. When the seeing is good, the planet doesn't break down easily; it keeps getting bigger and delivering more detail.
Like the disk, Saturn's rings are almost unchanging, "almost" because they do change their tilt. Saturn's inclination to the ecliptic (the plane of Earth's orbit) causes the aspect of the rings as seen from Earth to change as the planet moves along. Eventually, the rings appear edge on. Such a "ring plane crossing" last took place in 1995 and will happen again in 2009. At those times the rings, which are only about 100 meters thick, briefly disappear, which normally allows Earthly observers a good look at the disk and the planet's faint moons without the interfering glow of the ring system. Unfortunately, the precise moment of ring plane crossing won't be seen in 2009, since the planet will be in conjunction with the Sun at that time. The progression from open rings to closed rings to open again occurs in cycles of 13.7 and 15.2 years.
Moons? You want moons? Saturn's got 'em. An amazing retinue of 60 at last count. Most are small, but Saturn's largest satellite, Titan, at 5,150 km in diameter, is planet-sized. Titan, easily visible in a 90mm MCT, even has a thick atmosphere, which is dominated by nitrogen and traces of methane and other gases. The makeup of Titan's atmosphere gives the moon an orange color easily detectable in an 8-inch SCT. In addition to magnitude 8.4 Titan, four other Saturnian moons, Rhea (9.0), Tethys (10.3), Dione (10.4), and Enceladus (11.8), are easy to see in modest instruments. Rhea and Tethys definitely show in the ETX 90. The identities of the moons can be figured out with the aid of computer software or with the diagrams astronomy magazines print during Saturn's apparitions.
Saturn isn't as interesting a world for the CAT-equipped amateur as Jupiter, but it's beautiful. Its relative changelessness seems to fit the massive and brooding father of the gods. Even though you probably won't see anything new on this distant giant from night to night, you might not be able to stop yourself from turning your SCT to the ringed wonder any time it's over the horizon.
The Distant Giants For SCT-users, Uranus and Neptune, the Solar System's outer pair of planets, are the been there worlds, "been there" because there's not much to see. The only attraction for most amateurs is the simple satisfaction of having tracked down and viewed these objects in the telescope—you've been there.
Uranus Locating magnitude 5.8 Uranusisn't much of a problem for the CAT owner. The planet is even visible to the naked eye from somewhat dark sites and is obvious in a pair of binoculars, making it easy to find even with a non go-to CAT.
Though it's not a challenge to locate, Uranus can be a bit of a challenge to see. Its tiny disk, averaging 3.6 arc seconds across, makes it easy to mistake the planet for just another field star. Once the telescope is in the correct area, a fairly bright faintly greenish "star" should be obvious in the field. It won't show much indication of a disk at typical "finding powers" of 100x or below, but it will look distinctly nonstellar. Once the planet is centered, run the magnification up to at least 250x-300x to get as good a look as possible at this distant giant's minute globe.
Don't expect to see anything much, even at 500x. Uranus is a huge, slightly flattened gas giant 51,118 km in diameter at the equator, but it is very far away, in the vast outer reaches of the Solar System nearly 3 billion kilometers from the Sun. As at Saturn, there's a haze in the upper atmosphere that tends to mask atmospheric activity. What weather patterns there are are intrinsically faint. It's really cold out there, and there's very little energy to drive the planet's weather. Even Voyager 2, which flew by Uranus in 1986 at a distance of 81,500 km, didn't see much more than a featureless green globe.
Uranus is currently known to have 27 moons (named for Shakespearean characters) . Out of these 27, only 4 can be glimpsed visually in amateur telescopes, and only 2, Oberon and Titania, are doable (though not easily doable) with an 8-inch. Oberon is close to magnitude 14, and Titania is only slightly brighter at 13.7. The way to conquer the pair is to use high magnification to suppress background sky glow in the field, get Uranus out of the field with an occulting bar, like one used to hunt Phobos and Deimos at Mars, and to know exactly where to look. Sky & Telescope magazine's website includes a nice java applet that shows the locations of Uranus's moons for any date and time. No luck? A CCDcamera or a long-exposure-modified webcam will make quick work of the planet's five brightest satellites, which range down to "only" 15th magnitude. Uranus possesses a set of rings, but they are made of dark material and are virtually invisible from Earth—in amateur scopes, anyway.
Neptune Neptune is a lot like Uranus for telescopic observers—only more so. The bluish-green sea god is only slightly smaller in diameter than brother Uranus, being an immense 49,532 km across, but is even more distant from Father Sun at 4.5 billion kilometers. So, Neptune is both smaller and dimmer than Uranus. Its magnitude is 7.9, so we've left the realm of naked-eye objects; a pair binoculars or a small telescope is required even to see this distant planet as a "star." Neptune makes Uranus look big in a telescope. Its disk is a miniscule 2.9 arc seconds across, and high power, 300x and above, is needed to resolve it as a disk. Like Uranus, it isn't terribly difficult to locate, even without go-to. Also like Uranus, the problem is knowing for sure it's in the field. It does look somewhat non-stellar at modest powers but is not nearly as noticeable as Uranus. Use as much magnification as the seeing will allow to ferret out the disk.
What's visible on the 8t h planet? Neptune does have a more active atmosphere than Uranus, one probably driven by the heat generated in the planet's interior. Neptune has a much stronger internal heat source than Uranus—no one is quite sure why—and some visual observers and webcam imagers using large, long, focal length telescopes do occasionally seem to have seen some atmospheric detail—perhaps hints of the white clouds or the Great Dark Spot Voyager 2 imaged in 1989.
Like the other gas giants, Neptune is accompanied by a large train of moons: 13 are now known. Only one of these, Triton, is visible in amateur scopes. Surprisingly, this large world (2,700 km in diameter) is easier to see than any of the Uranian moons. At magnitude 13 Triton is fairly easy to pick out, if it's position relative to the planet is known. Most planetarium software will plot the current position of Neptune's big moon and will also allow the planet to be oriented to match the view in a CAT (right side up and mirror reversed). As for the other Neptunian satellites, the next brightest is Nereid at magnitude of 19.2. For this, you'd need to break out the CCD camera.
Pluto It will be up to you to decide whether to agree or disagree with the International Astronomical Union's decision to strip poor little Plutoof major planet status. One thing is clear: Pluto is in a whole other class compared to the larger members of the Sun's family, both in makeup and difficulty for observers. Unlike the outer gas giants that share its distant neighborhood, Pluto is a tiny ball of rock and ice—with the emphasis probably on ice. Once thought to be larger than Mercury, Pluto has been downsized every time we've learned more about it. The current accepted diameter of this moon-like world is a mere 2,274km. It would be tempting to dismiss this speck of a world as an escaped satellite of one of the gas giants, but current theories do not support that. Though it may be moon-like in size and composition, Pluto is actually the owner of three moons of its own, Charon, which is just a little smaller than Pluto itself, and two asteroid-sized chunks of ice and/or rock. All these moons are invisible in amateur scopes.
Pluto is also incredibly distant. On average, it is 5 billion kilometers from the Sun and subtends a bare .1 arc second in Earthly telescopes. Pluto is simply not resolvable as a disk by amateur scopes (large professional telescopes equipped with adaptive optics do have a shot). This tiny world is also dim at an average magnitude close to 14.0. That probably puts it out of range of smaller than 8-inch CATs, and it is not easy even in an 8-inch. Experienced observers can find Pluto with a C8, but the task becomes easier in an 11- or 12-inch. As with Neptune, the problem in this age of go-to is not so much finding Pluto but knowing it's been found. Worse, unlike Neptune, Pluto won't show even a tiny disk no matter how much power is thrown at it. To be sure it's is in the bag, check the eyepiece field against a detailed chart showing stars down to magnitude 14 and dimmer. The astronomy magazines usually print Pluto finder charts once a year, but the best bet is a computer program that will tailor the view to a particular scope and eyepiece. Even a highly detailed chart may not make it absolutely certain Pluto has been glimpsed. The time-honored verification method is to draw a quick sketch or make a CCDexposure of the field. Come back the next evening and check to see if the Pluto "candidate" has moved with respect to field stars. If so, success!
Why devote so much time to tracking down visually uninteresting Pluto? There's a special thrill in tracking down a world that until recent times—until Clyde Tom-baugh discovered it in the 1930s—was completely unknown. Finding Pluto is also a good test of both telescope and observing skills. Most of all, though, the pleasure comes from gazing upon a world that has been seen by few human eyes.
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